Falling Blind by Shannon K. Butcher The Sentinel Wars

They are the Sentinels. Three races descended from ancient guardians of mankind, each possessing unique abilities in their battle to protect humanity against their eternal foes: the Synestryn. Now a warrior must protect a strong-minded woman from an incredible evil….

A beautiful, independent Theronai, Rory Rainey knows that her attempts to be a warrior are futile—unless she can stop the demonic visions that torment her. Determined to be free, Rory sets out to find the mysterious guardian who can cure her, before she loses her sight—or her life. But Rory’s journey brings its own dangers when she is attacked by a pack of Synestryn sent by the demon lord Raygh.

Cornered and desperate, Rory narrowly escapes with the help of a sword-wielding stranger. Cain knows that Rory needs his protection to survive, just as he knows that her power is compatible with his own. And when Rory binds herself to Cain, the warrior knows he will do anything to keep her safe from the threat of the demon lord that pursues her….